MUSEUM OF COMPARATIVE ZOOLOGY. 101 



Fig. 43, 46, 47. Blastodermic cells ; to show some of the conditions presented by 

 the chromatine and nucleoplasm of their nuclei. 



" 44. A cell in the process of division, further advanced than the one repre- 

 sented in Fig. 42. 



" 45. A portion of Fig. 49 highly magnified to show the columnar nature of the 

 ectodermic cells and the complete differentiation of the mesoderm. 



" 48. Ectodermic cells with two nuclei from a late stage, during the infolding, 

 to form the ovary. 



" 49. Sagittal section through an embryo in the protozonite stage, X 110. 



Note. — In cutting and mounting, the section was artificially ruptured in 

 two places, but none of the blastoderm has fallen away. 



PLATE VIII. 



Fig. 50. Sagittal section through an embryo during reversion, showing stomo- 



dffium, tail-lobe, etc. X 110. 

 " 51, 52. Views of sagittal sections of the posterior region during reversion, to 



show the condition of tlie tail-fold. 

 " 63. Section of the tail region and apart of the dorsal region, to show the 



mesodermic somites of the rudimentary terga. The section is cut 



obliquely to the median plane. X 110. 

 " 5t. Sagittal section of the morphological tip (T) of the body near the close 



of reversion, to show the early condition of the proctodaeura. X 110. 

 " 65. Sagittal section of the hind pan of the body, to show the stercoral pocket 



(br. stc.) and the pre-stercoral tube {pr-stc). 

 " 56. Sagittal section of the hind part of the body, to show the trumpet-shaped 



condition of the pre-stercoral tube and the somatization of the body as 



indicated by the segmental grouping of the muscles {mu. 1 — mu. 6). 



PLATE IX. 



Fig. 57. A nearly horizontal section of the proctodaeura and the stercoral pocket 

 {br. stc), about three days before hatching, showing the columnar 

 epithelium and the narrow lumen of the proctodaeum. X 310. 



" 58. Sagittal sections of anus and stercoral pocket, about eight days after 

 hatching. X 110. 



" 59. About one half of a transverse section in the region of the " rudimentary 

 terga," to show tlie mesodermic somites of these dorsal elements and 

 their connection with the ventral portion of the mesoderm. X 100. 



" 60. Transverse section of an embryo near the beginning of reversion, pass- 

 ing through the stomodaeum and the 2nd pairs of legs. X 100. Com- 

 pare the separated nerve bands {fin.) with those of Fig. 62. 



" 61. A section from the same embryo as Fig. 59, showing entodermic cells 

 (en.) in the region of the tergal elements. 



" 62. Transverse section after completed reversion, through that pnrt of the 

 stomodasum wliich becomes the sucking stomacli. If shows the mus- 

 cles attached to sucking stomach -[ww. vrt.,mu. lat.), and the approxi- 

 mation of the nervous bands {(jn.). X 110. 



